The type and strength of magnetic material in different regions of a rotor used in electric motors may be varied. For example in U.S. Pat. No. 6,703,746, NdFeB is used to configure in the entire outer barrier of the rotor where it may be easily magnetized. However, in this patent high energy magnetic material in the middle section or the inner regions of the rotor are not exposed to a magnetizing field strong enough to fully magnetize high energy magnetic material. Consequently, low energy magnetic material is placed in areas of the rotor that are difficult to magnetize because low energy magnetic materials require a smaller magnitude magnetizing field than high energy magnetic materials. Accordingly, low energy magnetic material in the inner region may be fully magnetized. Low energy magnets in the inner region do not contribute to the air gap flux. However, the low energy magnets ensure bridge saturation, which is important to ensure high saliency corresponding to better performance. A non-magnetized high energy magnet in the inner region may contribute to a waste of valuable magnetic material as well as to inadequate bridge saturation. Inadequate bridge saturation can lower the rotor saliency and motor performance.
In U.S. Pat. No. 6,703,746, the high energy and low energy magnetic materials are injected in liquid form into separate cavities within the rotor with the high energy magnetic material being injected into outer cavities adjacent the periphery of the rotor and low energy magnetic material being injected into inner cavities closer to the axis of the rotor. When the liquid magnetic material solidifies, it is magnetized by the stator or other source of magnetization. In the arrangement of U.S. Pat. No. 6,703,746, each magnetic material in a cavity is consistent with an unmixed interface therebetween.
The design of an IPM motor is a series of tradeoffs to meet system objectives while minimizing unwanted side effects of the design. Torque ripple and losses within a rotor are two such unwanted side effects. Eddy current losses within a magnet in the motor contribute to the heating of the rotor. Segmenting sintered magnets along the axial length of the rotor is often employed to minimize the eddy current losses and thus rotor heating. Torque ripple can be minimized by careful shaping of the rotor magnets (thinner towards rotor surface) and/or skewing either the rotor or stator to smooth out air gap flux along the length of the motor. Skewing reduces the average torque production and complicates the manufacturing of the motor. Eddy current losses may be minimized by creating an additional mechanical bridge in the rotor so that a small portion of the cavity near the rotor surface can be left unfilled with magnet material. Since magnet material nearest the surface of the rotor contributes significantly to torque ripple and AC flux contributes to eddy currents, such techniques reduce both torque ripple and eddy current losses. Magnetic material nearer the surface of the rotor is easier to magnetize when the entire rotor is magnetized as one assembly, therefore placing stronger magnets near the rotor surface and weaker magnets deeper inside the rotor is a cost savings measure. There is a need, however, to make such arrangements of magnetic material as efficient as possible.